A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use EUV radiation which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.
Radiation may be produced using plasma. The plasma may be created, for example, by directing a laser at particles of a suitable material (e.g. tin), or by directing a laser at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a collector such as a mirrored incidence collector, which receives the radiation and focuses the radiation into a beam. Such a radiation source is typically termed a laser produced plasma (LPP) source.
In addition to radiation, the plasma of a plasma radiation source produces contamination in the form of particles, such as thermalized atoms, ions, nanoclusters, and/or microparticles. The contamination is output, together with the desired radiation, from the radiation source towards the collector and may cause damage to the incidence collector and/or other parts.
A radiation source may also output secondary radiation in addition to the desired radiation. For example, a EUV plasma radiation source may output secondary radiation having a wavelength selected from the range of 20-400 nm in addition to desired EUV radiation, most notably in the deep ultraviolet range (100-400 nm). Moreover, the secondary radiation may include EUV radiation as if a certain wavelength or range of wavelengths of EUV radiation is desired and other EUV radiation is in the output radiation that is not that certain wavelength of desired EUV radiation or in range of wavelengths of desired EUV radiation. Such secondary radiation may arise in a LPP radiation source due to the laser used to generate the plasma, the laser radiation having a wavelength longer than EUV radiation (often 10.6 μm wavelength radiation from a CO2 laser).
In lithography, it is desirable to improve spectral purity, i.e., to remove secondary radiation from the output beam to yield a higher proportion of desired radiation. For example, resist is sensitive to a wavelength of the secondary radiation, and thus the image quality may be deteriorated. Since the optics of a EUV lithographic apparatus has a high reflectivity (for example, for 10.6 μm wavelength secondary radiation from a LPP source), the secondary radiation may reach the substrate with significant power. Additionally or alternatively, the secondary radiation, particularly the laser radiation in a LPP radiation source, may lead to undesired heating of the patterning device, substrate, and/or optics.
Accordingly, it is desirable to provide, for example, a spectral purity filter for use in or with a radiation source, wherein secondary radiation may be removed completely or partially and/or contamination mitigation is effectively improved.